Thermal decomposition of organic compounds

ABSTRACT

THE PRESENT INVENTION RELATES TO AN IMPROVED PROCESS FOR THERMALLY DECOMPOSING ORGANIC COMPOUNDS IN CONTACT WITH ALLOY STEEL, E.G., THE CRACKING OF LOW MOLECULAR WEIGHT HYDROCARBONS IN HEAT-RESISTANT ALLOY STEEL FURNACE TUBES TO PRODUCE OLEFINS OR ACETYLENES. BY THE PRESENT INVENTION, HEAT-RESISTANT ALLOY STEEL COMPRISED OF AT LEAST 1% BY WEIGHT SILICON IS EMPLOYED, AND A CARBURIZATIONRESISTANT PROTECTIVE FILM COMPRISED OF SILICON OXIDATION COMPOUNDS IS FORMED THEREON. THE PROTECTIVE FILM IS FORMED BY PRETREATMENT OF THE ALLOY WITH STEAM PRIOR TO THERMALLY DECOMPOSING THE ORGANIC COMPOUND THEREIN AND/OR BY COMBINING A SMALL AMOUNT OR STEAM WITH THE ORGANIC COMPOUND SO THAT THE PROTECTIVE FILM IS FORMED DURING THE THERMAL DECOMPOSITION.

United States Patent r 3,704,333 THERMAL DECOMPOSITION OF ORGANIC COMPOUNDS Carl D. Spangler, .lr., Frederick J. Radd, and Louis H.

Wolfe, Ponca City, Okla, assignors to Continental Oil Company, Ponca City, Okla. No Drawing. Filed Aug. 20, 1970, Ser. No..65,665 Int. Cl. C07c 5/22 US. Cl. 260683 6 Claims ABSTRACT OF THE DISCLOSURE formed by pretreatment of the alloy with steam prior to thermally decomposing the organic compound therein and/or by combining a small amount of steam with the organic compound so that the protective film is formed during the thermal decomposition.

BACKGROUND OF THE INVENTION (1) Field of the invention The present invention relates generally to the thermal decomposition of organic compounds, and more particularly, but not by wa'y of limitation, to the thermal decomposition of organic compounds in contact with a heatresistant alloy steel.

(2) Description of the prior art Presently used processes for the thermal decomposition of organic compounds, such as the cracking or disproportionation of hydrocarbons, are commonly carried out in furnaces or heaters including alloy steel heating tubes.

For example, in the pyrolysis of ethane to yield ethylene,

the ethane is commonly charged to a pyrolysis furnace having alloy steel' heating tubes disposed thereinJWhile within the alloy steel heating tubes the ethane is subjected to high pyrolysis temperatures (1300 F. to '2000" F.) to yield the desired ethylene product.

While furnace tubes formed of highly heat-resistant alloy steel have been developed and used, continuing tube failure problems have been encountered in organic compound thermal decomposition processes. A major cause of such failures is carburization and corrosion of the tubes brought about by the extremely high temperatures encountered. Further, frequent cleaning and replacement of the tubes is occasioned by coke formation therein. Not only do coke deposits materially reduce the throughput capacity of a furnace tube, but also'hard deposits can cause tube failure during shutdown of a furnace due to the different coefi'icients of thermal expansion of the coke and the alloy steel. In addition, the entire capacity of the furnace is of course unavailable during tube cleanout, and the labor cost of such clean-out is not inconsequential.

It is well known that alloy steels containing nickel and chromium in varying proportions are useful in constructing highly heat-resistant heating tubes for thermal decomposition furnaces. However, while such heat-resistant alloy steel heating tubes ex'hibit longer performance lives than heating tubes formed of other materials, carburization and corrosion of the tubes and the deposition of coke therein has continued to be a problem. Carburization of the tubes, which is the diffusion of carbon into the alloy steel causing the formation of carbides, brings about the embrittling thereof. Once the steel has become embrittled it is susceptible to high rates of corrosion and catalytic site formation which in turn leads to the deposition of carbon in the form of coke thereon.

SUMMARY OF THE INVENTION The present invention relates to the thermal decomposi- -tion of an organic compound in contact with a heatresistant alloy steel wherein the alloy steel is comprised of at least 1% by weight silicon and the alloy steel is pretreated prior to the thermal decomposition by contact with steam at a temperature and for a period of time effective to form a protective film comprised of silicon oxidation compounds thereon.

It is, therefore, a general object of the present invention to provide an improved process for the thermaldecomposition of organic compounds.

A further object of the present invention is the provision of an improved process for the thermal decomposi- DESCRIPTION OF THE PREFERRED EMBODIMENTS It has been found by the applicants that the surfaces of heat-resistant alloy steel containing silicon in an amount of at least 1% by weight can be pretreated' by contact with steam at elevated temperatures, and that when such surfaces are subsequently placed inservice for'the thermal decomposition of organic compounds, such as cracking or: disproportionation, the' rate of J carburization and corrosion of such surfaces as well as the deposition of coke thereon are reduced. Further,iit' has been found that in lieu of the above pretreatment, or in combination therewith, small amounts of steam may be combined with the organic compound being thermally decomposed to bring about contact of the alloy steel surfaces therewith and the resultant reduction of carburization, corrosion, and coke deposition.

The present invention is based upon the discovery that contact of alloy steel surfaces containing silicon in an amount of at least about 1% by weight with an oxidizing agent such as steam brings about the formation of a protective film comprised of silicon oxidation compounds. Specifically, when contacted with steam, silicon contained in the alloy steel is oxidized and hydrated to form a protective film of hydrated silicon dioxide (SiO -nH O). Silicates of other elements contained in the'alloy steel are also formed which contribute to the protective film, such as manganese silicate (MnSiO Heat resistant iron-chromium-nickel alloys containing at least about 1% silicon by weight are preferred for use in the present inventionflHowever, other chromium or chromium-nickel type alloys may be utilized so long as silicon is'present' therein in an amount of at least about the silicon containing alloy can be used as a lining in another alloy which will provide the ductility and toughness required, the silicon liner can consist essentially of silicon only. I

In a preferred embodiment of the present invention,

4 EXAMPLE 1 (1) Laboratory tests were conducted to determine the effectiveness of the present invention in reducing carburization, corrosion, and coke deposition on alloy steel suralloy steel furnace tubes to be used for the thermal defaces 111 the thermal deeompesltlon 0f f P P- composition of organic. compounds containing at least flucfi ethylene. The alloys tested were tWO SlllCOn contam- 1% by weight silicon are pretreated by contact with steam f eemmel'elally avallable heat"l'eslstant east alloys P prior to being placed in service. Pretreatment at elevated ly Sultable hlgh temperature Servlee- The cheml temperatures of at least about 900 F. is generally recal compositions of the tested alloys are given in Table I quired, although temperatures up to those which the hereinbelow.

TABLE I [Chemical compositions of silicon containing alloy steels tested] Alloy Manga- Phos- Molybtype Silicon Carbon nese phorus Sulfur denum Chromium Nickel Iron Sample Number:

1.0 0 20.6 2.0 0. 04 0. 04 0.5 24.0-28.0 '18. 0-22.0 Balance. 1.5 0.2-0.6 2.0 0. 04 0. 04 0.5 24. 028.0 18.022.0 Do. 2.0 0. 2-0.0 2.0 0. 04 0.04 0.5 24.0-28.0 18.0-22.0 Do. 2.5 0. 35-0. 75 2.0 0. 04 0.04 0.5 17.0-21.0 37. p-41.0 Do.

1 Designations of the Alloy Casting Institute, 300 Madison Ave., New York, N.Y.

. tube is safely capable of undergoing are preferred.'The

duration of the treatment is subject to variation, although lengthy periods are not advantageous. Thus, exposure of the alloy surface to steam contact for at least one minute is required. An exposure for an hour or more is-preferred. Once the pretreatment has been completed and the protective film of silicon oxidation compounds is formed thereon, the thermal decomposition may be commenced.

As an alternate to the above described pretreatment, a protective film of silicon oxidation compounds may be formed during the thermal decomposition of an organic compound by combining small amounts of steam with the organic compound being processed. Thus, as the thermal decomposition is carried out the surfaces of the silicon containing alloy steel will be contacted by the steam and the protective film of silicon oxidation compounds formed thereon. Large quantities of steam are not required for this purpose, and it has been found that traces of steam combined with the organic compound are suflicient to'inhibit carburization. Preferably, however, steam is added to the organic compound in an amount of from about 0.01 pound steam per pound of organic compound to about 1.0 pound of steam per pound of organic compound.

The alloy steel pretreatment described above may be advantageously combined with theaddition of small amounts of steam to the organic compound being thermally decomposed to reduce the carburization, corrosion, and deposition of coke on the alloy steel. That is, after the protective film of silicon oxidation compounds has.

been formed by the steam pretreatment, the film may be maintained during the carrying out of the decomposition reaction by the addition of small amounts of steam to the organic compound.

The present invention is advantageously used for thermal, as opposed to catalytic processes. Exemplary of such processes are thermal disproportionation and cracking, e.g., cracking of hydrocarbons such as ethane, propane, etc. to produce olefins or acetylenes. By alloy steel is meant those steels which have become prevalent for furnaces recently because of the increased demand for metals capable of withstanding high temperatures and pressures. Such alloys, generally, but not by way of limitation, include major portions of iron, chromium, and nickel, and can include minor amounts of such elements as aluminum, manganese, phosphorus, sulfur, molybdenum, tantalum, titanium, and zirconium in addition to silicon which, as hereinbefore described, must be present in an'amount of at least about 1% by weight.

The invention will be further demonstrated by the following-examples: I a

In the test, ethane-hydrogen sulfide mixtures similar .to those actually encountered in plant operations, the first free of water and the second water-saturated, were charged to a special thermal decomposition reactor at a flow rate of 1 liter per minute. The reactor was held at a temperature of 1630 F. and a pressure of 15 p.s.i.a. The alloys tested were disposed in coupon form within the reactor, and after thermal decomposition of the ethane mixtures to form ethylene for periods of 10 hours, the coupons were tested for carburization. The percent conversion of the ethane to coke was also noted for each ItJest period. The results of these tests are shown in Table II elow.

TABLE II {Effect of steam contact on silicon containing alloy steel during thermal decomposition of ethane at 1630 F. and 15 p.s.i.a.]

From the above it may be seen that the depth of carburization of the silicon containing alloys as well as the deposition of coke thereon was substantially reduced by contact of the alloy with steam and the formationof the hereinbefore described protective film.

EXAMPLE 2 Additional laboratory tests identical to those described in Example 1 above were made, except that the quantities of steam combined with the ethane-hydrogen sulfide mixtures were varied. For each test, the depths of carburization of the alloy coupons tested were determined. The results of these tests are given in Table III below.

TABLE III [Efiect of steam contact at various rates on silicon containing alloy steel during thermal decomposition of ethane at 1630 F. and 15 p.s.i.a.]

Alloys from Table I, depht of carburization Steam com- 01 alloy tested bined with (mini-inches) Quantity of ethane (lbs. 1128 in ethane steam/lb. Sample 3 Sample 4 (p.p.m.) ethane) HK alloy HU alloy From the above it may be seen that only small quantities of steam are required to form the protective film of the present invention on silicon containing alloy steel.

EXAMPLE 3 Five identical new cracking furnaces were erected, the tubes being formed of the HK alloy described as sample Number 3 in Table I above. The tubes of one furnace were pretreated with steam at about 1750 F. for about fifteen hours, while the other furnaces were not subjected to any treatment. All five furnaces were then placed on duty cracking ethane to produce ethylene. During the initial approximately five months of operation of these furnaces, the pretreated furnace was on stream about 82.3% of the time, while the average of the other four was about 74.7%.

EXAMPLE 4 A cracking furnace having tubes being formed of the HK alloy described as sample Numbers 1 and 2 in Table I above was pretreated with steam at about 1750 F. for about 15 hours prior to placing in ethane cracking service. After approximately five months of operation these tubes were examined for carburization depth. In the HK alloy having 1.0% silicon the carburization depth was one-eighth of an inch and the HK alloy having 1.5% silicon showed no noticeable signs of carburization.

Although this invention has been described with respect to particular embodiments thereof, it is not to be so limited, as changes and modifications may be made therein which are within the spirit and the scope of the invention as defined by the appended claims.

What is claimed is:

1. In the thermal cracking of a hydrocarbon in contact with heat-resistant alloy steel, the improvement comprising:

said heat resisting alloy steel being comprised of at least 1% by weight silicon; and

pretreating said alloy prior to said thermal cracking by contacting it with steam at a temperature and for a period of time effective to form a protective film thereon of hydrated silicon dioxide.

2. The process of claim 1 wherein said temperature is at least about 900 F. and said time is at least about one minute.

3. In the thermal cracking of a hydrocarbon in contact with a heat-resistant alloy steel, the improvement comprising:

said heat-resistant alloy steel containing at least 1% by weight silicon; and

combining at least about 0.01 pound of steam per pound of hydrocarbon with said hydrocarbon so that during the thermal cracking thereof said alloy is contacted with said steam thereby forming a protective film thereon comprised of hydrated silicon dioxide.

4. In the thermal cracking of a hydrocarbon in contact with a heat-resistant alloy steel, the improvement comprising:

said heat-resistant alloy steel containing at least 1% by weight silicon;

pretreating said alloy prior to said thermal cracking by contacting it with steam at a temperature and for -a period of time effective to form a protective film thereon comprised of hydrated silicon dioxide; and combining at least about 0.01 pound of steam per pound of hydrocarbon with said hydrocarbon so that during the thermal cracking thereof said alloy is contacted with said steam thereby maintaining said protective film of hydrated silicon dioxide thereon.

5. The process of claim 4 wherein said temperature is at least about 900 F. and said time is at least about one minute.

6. The process of claim 4 wherein said hydrocarbon is ethane.

References Cited UNITED STATES PATENTS 3,248,441 4/1966 Soderquist et al. 260669 3,536,776 10/1970 Lo 260683 3,176,047 3/1965 Braconier et al. 260683 3,019,271 1/1962 Braconier et al. 260683 1,759,605 5/1930 De Vries 23252 A 1,815,428 9/1927 Black 23252 A DELBERT E. GANTZ, Primary Examiner J. M. NELSON, Assistant Examiner US. Cl. X.R.

260679 R; 20848 R; 23252 A 

